Since 1994, it has been obligatory for vehicles manufactured in the United States to be fitted with an On Board Diagnosis (OBD) II fault diagnosis device to prevent worsening of noxious exhaust emissions due to vehicle aging. One of these devices, which detects misfiring of an engine, is disclosed in Tokkai Hei 4-113244. This device is provided with a magnetic pickup that reacts to the passage of teeth on a ring gear connected to the crankshaft of the engine, detects variations in the rotation speed of the ring gear from the output signal, and thereby tests whether or not misfiring has occurred.
The rotation speed of the ring gear largely varies depending on, for example, whether or not drive torque is being transmitted from the engine output shaft to the vehicle drive shaft, so to avoid false conclusions, the test is performed when for example the neutral switch is in the OFF position.
When the transmission is in the neutral position, i.e. when rotation is not being transmitted from the engine output shaft to the vehicle drive shaft, the neutral switch outputs an ON signal, and in other cases it outputs an OFF signal.
When misfiring is diagnosed only when the neutral switch is OFF, it is necessary to determine that the neutral switch is not faulty before performing an analysis of ignition failure.
This determination of whether the neutral switch is faulty is made by determining whether or not an ON signal is output by the neutral switch during a period from when the starter switch changes over from ON to OFF, i.e. from engine startup, to when the misfiring diagnosis is performed.
When the engine is started, the engine output shaft and the vehicle drive shaft are not connected. i.e., the transmission should be in the neutral position. Hence, if the neutral switch does not switch ON even once during the period from engine startup to when the misfiring diagnosis is performed, it is determined that the neutral switch has a fault.
However, when the engine is restarted while the vehicle is running (e.g., coasting downhill), the transmission may be in the driving range from startup to when the misfiring diagnosis is performed.
In this case, if the neutral switch fault test is made by the aforesaid determining algorithm, it is incorrectly determined that the neutral switch has a fault even when it is functioning correctly.
A lockup switch may be used instead of the neutral switch in order to detect the state of torque transmission from the engine output shaft to the vehicle drive shaft.
The lockup switch outputs an ON signal when the engine output shaft and the transmission are directly connected mechanically without the intervention of a fluid clutch, on torque converter and in other cases it outputs an OFF signal.
However, if an algorithm is used which determines that the lockup switch is faulty if it does not output an OFF signal even once from startup to when the misfiring diagnosis is performed, the determination will again be incorrect as in the case of the neutral switch.